Blue-flowering shrubs are unusual and stunning in the autumnal garden. Two worthy of trial are caryopteris and ceratostigma.
Caryopteris is a genus of low-growing, spreading, deciduous shrubs, with small, softly-textured leaves; it is also known as blue spiraea. The genus name comes from the Greek karnon for nut and pteron, wing, referring to the winged fruit. The shrub produces masses of blue or violet-blue flowers in fall. The flowers have one petal larger than the others and fringed; hence the common name “bluebeard.”
Caryopteris is not always hardy in exposed areas and is susceptible to frosts; hybrids have been found to be hardier. It likes sun, a not-too-heavy soil, and lime. There are six species of caryopteris native to Asia but many hybrids are commonly cultivated. Caryopteris x clandonensis (blue mist), a hybrid developed in England, has become quite popular. Caryopteris incana (common bluebeard) is also frequently grown; however, it is hardy only to 10 degrees F.
Caryopteris can be planted in March or April and should be protected from frosts the first winter. It can be pruned fairly hard the following April, with old growth cut back to two buds from the base. Seed germinates easily but plants are not always true to color. Propagation may be by softwood cuttings taken in July and rooted in a greenhouse, or hardwood cuttings rooted in a cold frame in November.
Ceratostigma is a small genus of deciduous shrubs (and perennial groundcovers) that produce very blue, round flowers. The genus name comes from Greek keras, a horn, and stigma, referring to branches of the stigma. These plants are natives of Africa and the Far East, and are related to the greenhouse plumbago. The common name ‘leadwort’ suggests that they were used at one time to treat lead poisoning.
Ceratostigma willmottianum (Chinese plumbago) is a three-foot tall shrub that provides a lovely blue background for perennial borders. It is cold hardy to 0 degrees F. It was introduced from China by Ernest Wilson, a famous plant collector who sent the seed to England.
Leadwort is forgiving about soil, although it does better in moist and well-drained sites. It can be planted in spring in sheltered places or sunny rock gardens. Propagation is by detaching rooted offsets in spring, or from stem cuttings in late spring or early summer.
Sources
Geneve, Robert. 2000. A Book of Blue Flowers. Timber Press, Portland, Oregon.
Hunt, Peter, Ed. 1970. The Marshall Cavendish Illustrated Encyclopedia of Gardening. Marshall Cavendish, New York.
